Separatists calling for the breakup of Nigeria have crossed a red line and the country's unity is not negotiable, President Muhammadu Buhari said on Monday in his first speech since returning home after three months of medical leave in Britain.

Ethnic tensions have surfaced in the last few months amid calls for a separate southeastern state, known as Biafra, while some in the restive southern Niger Delta oil hub have called for independence and Boko Haram militants seek a state in the northeast.

In a televised speech, broadcast two days after his return on Saturday, Buhari said he followed events in Nigeria on a daily basis during his absence and was concerned by discussions over the possible dissolution of the country.

"I was distressed to notice that some of the comments, especially in the social media have crossed our national red lines by daring to question our collective existence as a nation," said Buhari.

"Nigeria's unity is settled and not negotiable. We shall not allow irresponsible elements to start trouble," he said, adding that some ethnic violence was "fueled by political mischief makers."

Buhari, a 74-year-old northern Muslim who took office in May 2015, handed over power to his deputy, Christian southerner Yemi Osinbajo, when he traveled to Britain on May 7 for treatment of an unspecified ailment.

Osinbajo held talks in June with regional leaders in an effort to quell tension after activists demanded the eviction of Igbo people from the north over their calls for the creation of a separate Biafra state.

The tensions echoed the build-up to a civil war over the secession of Biafra in 1967 which ended with Nigeria's victory in 1970, after an estimated 1 million people died.

Buhari also said Nigeria's security agencies should not let successes achieved against extremist group Boko Haram in the last 18 months "be a sign to relax", adding that Nigeria would "reinforce and reinvigorate" the fight against the insurgents.

Bomb attacks and raids by Boko Haram in the northeast have increased in the last few months, killing at least 170 people since June 1, according to a Reuters tally. Buhari had said in December 2016 that the group's main base had fallen.

Nigeria, an OPEC member which has Africa's biggest economy, is in its first recession in 25 years which was largely caused by low oil prices and the impact of attacks on energy facilities last year in the Niger Delta.

Buhari said it was in the interest of Nigerians to come together to face the challenge of economic security.

(Source: Reuters)